The present invention relates to a driving system for electric vehicles and, more particularly, to a driving system for electric vehicles having a plurality of batteries.
In a conventional hybrid-type electric vehicle, an internal combustion engine drives an electric power generator which in turn supplies electric power to both of a main battery of high voltage (e.g., 300 V) and a vehicle drive motor. The main battery (high voltage battery) supplies the electric power to a starter motor which in turn starts the internal combustion engine. This starter motor may use the above electric power generator for engine starting.
In JP 54-9763, an accessory battery (low voltage battery) supplies electric power to an electronic control circuit which includes a microcomputer and a logic interface. The control circuit controls a three-phase inverter circuit for driving a vehicle drive motor.
In such an electric vehicle driving system having the accessory battery, the vehicle drive motor can be driven by a higher voltage power than other normal accessory devices. Thus, it is advantageous that the drive motor can be operated with less loss of electric power and its drive unit can be reduced in size, while vehicle accessory devices can be operated by the lower voltage power which has less voltage fluctuation.
In the above electric vehicle driving system having a plurality of batteries, the accessory battery also supplies the electric power to a driver circuit which switches on and off the switching devices in the three-phase inverter circuit for controlling the vehicle drive motor. It is therefore proposed to connect an input-output isolating type DC-DC converter between the accessory battery and the power supply terminals of the driver circuit, and to connect photo couplers between the control circuit and the control terminals of switching devices of the driver circuit.
As shown in FIG. 3 in more detail, the electric vehicle driving system having the accessory battery comprises a main battery 1, an accessory battery 2, an electronic driver circuit unit 3, and a vehicle drive motor 4. The driver circuit unit 3 includes a three-phase inverter circuit 31, a photo coupler circuit 32, a control circuit (CC) 33, a driver circuit power source 34 and a relay 35.
The three-phase inverter circuit 31 includes insulated-gate bipolar transistors (IGBTS) 3a-3f, diodes (not shown) connected in parallel and in opposite polarity with the IGBTs 3a-3f, and gate control circuit (not shown) which controls IGBTs 3a-3f. The gate control circuit thus constitutes together with the photo coupler circuit 32 a driver circuit for the three-phase inverter circuit 31.
The photo coupler circuit 32 includes photo couplers 32a-32c which switch on and off IGBTs 3a-3c respectively, and photo couplers 32d-32f which switch on and off IGBTs 3d-3f respectively. The IGBTs 3a-3c are used as high voltage side switches, while IGBTs 3d-3f are used as low voltage side switches.
The driver circuit power source 34 is a switching-type voltage-reducing DC-DC converter which includes a transformer 340 and rectifier circuits 34a-34d. The rectifier circuits 34a-34c supply voltages to the gates of the IGBTs 3a-3c through photo couplers 32a-32c, respectively. The rectifier circuit 34d supplies a voltage to the gates of the IGBTs 3d-3f through photo couplers 32d-32f.
A switching transistor 36 is connected between the control circuit 33 and the transformer 34. The control circuit 33 switches on and off the switching transistor 36 at a fixed frequency to apply an alternating current (AC) voltage to the primary winding of the transformer 340.
In the above system, a pair of high voltage side power lines 5 supply high voltage electric power from the main battery 1 to the driver circuit unit 3, while a pair of low voltage side power lines 7 supply low voltage electric power from the accessory battery 2 to the driver circuit unit 3. Three motor driving power lines 6 connect the driver circuit unit 3 to the vehicle driver motor 4. Both of the high voltage power supply lines 5 and the motor driving power lines 6 are shielded-type to reduce radio noise.
This system has the following disadvantages.
The vehicle drive motor 4 has a large reactance. When the IGBTs 3a-3f in the three-phase inverter circuit 3 turn on and off repeatedly, this reactance will cause large switching noise voltages to be imposed on the low voltage power lines 7 of the accessory battery 2 from the control terminals of the IGBTs 3a-3d through the driver circuit power source 34.
The driver circuit turns on and off repeatedly to drive the IGBTs 3a-3f by receiving the electric power from the driver circuit power source 34 and power-amplifying signals from the photo coupler circuit 32. This switching operation also cause large switching noise voltages to be superimposed on the low voltage power lines 7 of the accessory battery 2 through the driver circuit power source 34.
The driver circuit power source 34 which is the switching-type voltage-reducing DC-DC converter requires a periodic switching of the switching transistor 36. Further, the output voltage of the driver circuit power source 34 includes ripples. As a result, the potential of the low voltage power lines 7 for supplying the electric power to the driver circuit power source 34 fluctuate periodically, causing switching noises to be superimposed on the low voltage power lines 7 in the similar manner as the switching noises caused by the IGBTs 3a-3f of the three-phase inverter circuit 31.
In the control circuit 33, its high power driving transistors providing an output interface circuit in particular turns on and off repeatedly. This causes fluctuation in the potential of the low voltage power lines 7 which supplies the electric power to the control circuit 33, resulting in the similar noise problem as well.
It is likely that the above various switching noise voltages superimposed on the low voltage power lines 7 cause other electronic circuits and electronic devices connected to the low voltage power line 7, and that the voltage fluctuation of the low voltage power lines 7 shorten the longevity of the accessory battery 2. Thus, the low voltage power lines 7 must be electromagnetically shielded to suppress generation of radio noises from the low voltage power lines 7.
When the accessory battery 2 runs down because of long non-use, electric power supply from the accessory battery 2 to the control circuit 33, driver circuit power source (switching-type voltage-reducing DC-DC converter) 34 and the driver circuit is disabled. Thus, it may occur that the vehicle drive motor 4 is disabled to operate even under the condition that the main battery 1 still has a sufficient electric power for the vehicle drive motor 4.